1. Field of the Invention
This invention relates to a novel polymerizable liquid crystal compound, a liquid crystal composition comprising the same, and an optical film obtained from the liquid crystal composition. The optical film is used by integrating it as various kinds of retardation film (optical compensation film) or as a cholestericpolarizer into a liquid crystal display (LCD).
2. Description of the Related Art
Conventionally, an optical film such as optical compensation plate used in liquid crystal displays makes use of a birefringent film produced by subjecting a polymeric film to stretching treatment, but recently a liquid crystal alignment film comprising a liquid crystal polymer or a liquid crystal compound having polymerizable functional groups has been developed in order to achieve an advanced alignment which cannot be achieved by stretching treatment, that is, alignment such as tilted alignment and twisted alignment.
Further, a cholesteric polarizer utilizing the selective reflection properties of a liquid crystal alignment film (selective reflection film) obtained from a cholesteric alignment composition comprising a liquid crystal polymer or a polymerizable liquid crystal compound combined with a chiral compound is also practically used. Among the selective reflection properties, the selective reflection central wavelength xcex is represented by the equation xcex=nxc3x97P wherein n is average refractive index, and P is cholesteric pitch, and the selective reflection wavelength width xcex94xcex is represented by the equation xcex94xcex=xcex94nxc3x97P wherein An is extraordinary light refractive index (ne) minus ordinary light refractive index (no). Accordingly, the selective reflection wavelength width xcex94xcex is represented by the equation of xcex94xcex=xcex94nxc3x97P=xcex94n/nxc3x97xcex. That is, the selective reflection wavelength width xcex94xcex is determined by the xcex94n/n of the film material.
When the selective reflection film is used as a cholesteric polarizer in LCD, it should cause selective reflection in the visible light range, but usually the selective reflection wavelength width xcex94xcex of one selective reflection film is narrower than the visible light zone, and thus a plurality of selective reflection films are laminated in order to broaden the selective reflection wavelength width xcex94xcex. Accordingly, when a material having a narrow selective reflection wavelength width xcex94xcex is used in a selective reflection film, the number of films laminated should be increased thus resulting in the problem of lower productivity. Under these circumstances, there is demand for a material having a broad selective reflection wavelength width xcex94xcex, that is, a material (e.g. a polymerizable liquid crystal compound) having a large value of xcex94n/n. When sandwiched between two alignment films in LCD, a known polymerizable compound or composition having a large value of xcex94n/n is aligned; however, when applied onto an alignment film, the compound or composition is poor in orientability or in solubility and coating properties, thus failing to form a film. Accordingly, a practically usable selective reflection film cannot be easily obtained.
An object of this invention is to provide a polymerizable liquid crystal compound having a large value of xcex94n/n, excellent in coating properties on an alignment film and easily aligned, as well as a liquid crystal composition comprising the polymerizable liquid crystal compound. Another object of this invention is to provide an optical film consisting of a liquid crystal alignment film obtained from the polymerizable liquid crystal compound or the liquid crystal composition, a selective reflection film selecting from the optical film, and a liquid crystal display comprising the optical film integrated therein.
To solve the problem described above, the present inventors made extensive study on various polymerizable liquid crystal compounds, and as a result, they found the following polymerizable liquid crystal compound, thus arriving at completion of this invention.
That is, this invention relates to a polymerizable liquid crystal compound having at least one polymerizable reactive group and showing nematic liquid crystal properties, wherein said compound has a xcex94n/n value of 0.14 or more wherein n is average refractive index and xcex94n is extraordinary light refractive index (ne) minus ordinary light refractive index (no), and is aligned by application onto an alignment film.
The polymerizable liquid crystal compound has a xcex94n/n value as high as 0.14 or more, and is excellent in coating properties and alignment properties on an alignment film. It is noted that xcex94n/n is equal to xcex94xcex/xcex, and the value of xcex94n/n can also be determined from the selective reflection central wavelength (xcex) and the selective reflection wavelength width (xcex94xcex) of the liquid crystal alignment film (selective reflection film) obtained from the polymerizable liquid crystal compound.
Further, this invention relates to a liquid crystal composition comprising the polymerizable liquid crystal compound.
Further, this invention relates to a liquid crystal composition comprising the polymerizable liquid crystal compound and a chiral compound.
The polymerizable liquid crystal compound can also be used in a liquid crystal composition, and upon incorporating a chiral compound for bringing about cholesteric liquid crystal properties, the polymerizable liquid crystal compound serves as a composition capable of cholesteric alignment.
The liquid crystal composition of this invention can further comprise a multifunctional (meth)acrylate compound having two or more (meth)acryloyloxy groups.
By incorporating the multifunctional (meth)acrylate compound having two or more (meth)acryloyloxy groups into the liquid crystal compound, the cured product thereof (liquid crystal alignment film) can further improve heat resistance and solvent resistance.
The liquid crystal composition comprising the polymerizable liquid crystal compound, the multifunctional (meth)acrylate compound having two or more (meth)acryloyloxy groups and a chiral compound can be used as a liquid crystal composition for producing a cholesteric liquid crystal film.
The liquid crystal composition comprising the polymerizable liquid crystal compound, the multifunctional (meth)acrylate compound having two or more (meth)acryloyloxy groups and a chiral compound can be used as a cholesteric liquid crystal composition for circular-dichroic optics.
Further, this invention relates to an optical film consisting of a liquid crystal alignment film obtained by coating an alignment film with the polymerizable liquid crystal compound or the liquid crystal composition, heat-orienting it and allowing (meth) acryloyloxy groups thereof to react in order to fix the alignment structure.
The liquid crystal alignment film obtained by orienting the polymerizable liquid crystal compound or liquid crystal composition and fixing the alignment thereof is also useful as various kinds of optical film. Out of the optical films described above, the optical film obtained from the liquid crystal composition comprising the polymerizable liquid crystal compound and a chiral compound is useful as a selective reflection film. Further, the selective reflection film can be laminated on a retardation film to form a cholesteric polarizer. Further, the optical film can be used after it is integrated in a usual manner into LCD.
As the xcex94n/n is increased, the selective reflection wavelength width xcex94xcex is made broader, and the number of film layers laminated for a broader range of the selective reflection film can be reduced. When the xcex94n/n is not less than 0.14, the visible-light region of from 400 to 700 nm can be covered by 4 layers. When the xcex94n/n is not less than 0.18, the visible light region can be covered by 3 layers.